A number of solid particulate materials can become contaminated with oily materials, requiring remediation. Solids obtained from well drilling operations, produced solids from oil production, contaminated soils, or other solid agents (inorganic and organic) can be exposed to oil, whether deliberately or inadvertently. Cleaning the solid materials and providing an environmentally-friendly disposal system are challenges in the industry.
As an example, cuttings produced in during well-drilling can be contaminated with oil, requiring remediation. Cuttings are shavings of earth-based solids formed by the action of the drill within the formation. Cuttings can include materials such as clay, shale, silt, ash, fragmented rock, minerals, soil, dirt, mud, sand, gravel, and the like.
In well drilling, a drilling fluid, also called “drilling mud,” is circulated through a pipe to lubricate the drill bit during drilling. There are two general categories of drilling mud: water-based mud, and oil-based mud. Formulations for oil-based muds comprise synthetic oils or other synthetic lubricating fluids, as well as refined and traditional oils. An oil-based drilling fluid typically contains oil or a synthetic hydrocarbon as the continuous phase, and may further contain water dispersed in the oil phase by emulsification, forming a water-in-oil or “invert” emulsion.
Oil or synthetic fluid-based muds are useful for drilling in formations such as swelling or sloughing shales, salt, gypsum, anhydrite or other evaporate formations, hydrogen sulfide-containing formations, and they may be useful when drilling under hot conditions (e.g., where temperatures within the borehole approach or exceed the boiling point of water). For deep-sea drilling (e.g., for wells at water depth greater than about 1500 feet deep), for example, oil-based drilling muds are preferred because of the extreme depth that these wells reach, with extreme temperature and pressure conditions. It is understood that temperatures in subterranean formations generally rise about 1 degree C. per hundred feet of depth, so that higher temperatures will be encountered as wells are drilled deeper. Oil based muds generally allow faster penetration rates during drilling, allowing a faster well completion. This benefit is important in off-shore drilling, where the operating expenses of a drilling platform can be measured in the hundreds of thousands of dollars per day. Oil based muds allow horizontal or high angle drilling with less risk of structural collapse of the wellbore. Oil based drilling muds typically comprise an oil-continuous phase, and may also contain as a discontinuous phase various aqueous solutions (such as sodium, potassium or calcium brines), along with other additives (e.g., rheology modifiers like oleophilic clays, weighting agents like barium sulfate, fluid loss control agents and the like).
When oil-based muds are used for drilling, the cuttings formed during the drilling process are coated with an oily residue derived from the drilling mud itself. The cuttings are carried back to the surface by a return flow of drilling mud. Upon their return to the surface, the cuttings are typically cleaned to remove the drilling mud that coats them. Conventional methods like shale shakers, mud cleaners, and centrifuges can separate the drilling mud from the cuttings so that the mud can be reused. The cleaned cuttings must then be disposed of, typically by discharging into the surrounding environment. Disposal is problematic, though, because the cuttings are still contaminated by a residual amount of hydrocarbons from the drilling fluid and hydrocarbons from the formation. This problem is exacerbated on offshore drilling platforms, where disposal of contaminated cuttings into the ocean environment can be hazardous to marine life.
Current regulations in certain oil-producing jurisdictions (e.g., the North Sea legislation OSPAR 2000/3) require that drill cuttings contain less than 1% total petroleum hydrocarbon (TPH) in order to be disposed on-site. Current on-site mechanical methods of mud and cutting separation reduce the oil content to 3-10% TPH. If the cuttings cannot be treated to conform to the regulatory requirements of less than 1% TPH, they must be disposed of in environmentally-protective ways, typically by removing them from the drilling site for specialized disposal. Disposal methods in accordance with such regulations can be costly and capital-intensive. There remains a need in the art, therefore, for an efficient method for reducing the TPH content of drill cuttings to allow their disposal off-shore and on-shore in conformity with environmental protection regulations.